Ethylene-vinyl alcohol copolymer, which may henceforth be abbreviated as EVOH, is a useful macromolecular material superior in oxygen barrier property, oil resistance, antistatic property, mechanical strength and the like, and is in wide use as various types of packaging material such as films, sheets and containers. Although EVOH pellets are molded into various molded article by various methods, they are often melt-molded by, for example, extrusion molding or injection molding. However, in general, when molding an EVOH resin, it is necessary to set the melting temperature to be 200° C. or higher. Therefore, EVOH containing no additives is liable to deteriorate when being melt molded and it accordingly may cause deterioration in product quality due to formation of fish eyes or hard spots in products. In addition, it is necessary to add some additives in order also to improve the interlayer adhesiveness when using it with other resins while laminating them.
In a pamphlet of WO 99/05213 (U.S. Pat. No. 6,174,949), disclosed is an EVOH resin composition containing a boron compound as an essential component, acetic acid as an optional component, and at least one compound as an essential component selected among acetic acid salts and phosphoric acid compounds, the content of each component based on 100 parts by weight of EVOH being 0.001 to 1 part by weight in terms of boron for the boron compound, 0 to 0.05 part by weight for acetic acid, 0.001 to 0.05 part by weight in terms of metal for the acetic acid salt, and 0.0005 to 0.05 part by weight in terms of phosphate group for the phosphoric acid compound. This resin composition is reported to be an EVOH resin composition having been improved in long-run workability, appearance and interlayer adhesiveness. The publication discloses that the purpose of the incorporation of the acetic acid salt is to improve the long-run workability or the interlayer adhesiveness.
JP-A-164059/2001 (EP-A-1090953) discloses an EVOH resin composition which is characterized in that, when it is heated to melt, its MFR shows specific behavior and that it contains from 50 to 500 ppm of carboxylic acid having a molecular weight of less than 75, from 50 to 500 ppm, in terms of metal element, of an alkali metal salt, from 10 to 120 ppm, in terms of metal element, of an alkaline earth metal salt, from 10 to 200 ppm, in terms of phosphate radical, of a phosphoric acid compound and from 50 to 2000 ppm, in terms of boron element, of a boron compound. This resin composition is reported to be an EVOH resin composition which is superior in appearance and in long-run workability at the time of its melt molding, less suffers yellowing when being recycled, and shows a superior interlayer adhesiveness when being fabricated into a laminate. In this invention, the alkali metal salt and the boron compound are added for improving the interlayer adhesiveness and for improving the long-run workability, respectively.
As a typical method for producing pellets of EVOH containing the additives, a method in which water-containing EVOH pellets are contacted with an aqueous solution containing the additives is disclosed. According to this method, it is easy to control the amounts of minor components contained in the EVOH pellets through an adjustment of the solution concentration and, therefore, it is possible to obtain pellets of stable quality by contacting them with the aqueous solution and then drying.
As mentioned above, the addition of an alkali metal salt to an EVOH resin in order to improve the interlayer adhesiveness has been done conventionally. The alkali metal salt is added typically in the form of an acetate. In many cases, acetic acid, which is not in the form of a salt, is also added simultaneously. Such an EVOH resin composition containing an acetate radical, however, may emit an acetic acid smell. One of the main applications of EVOH resin compositions is food packaging containers. In the market, EVOH resin compositions emitting smell as less as possible have been sought. In addition, EVOH resin compositions having a more improved melt stability and a superior long-run workability have been awaited.
On the other hand, in many cases where EVOH resin composition pellets are produced by contacting water-containing EVOH pellets with an aqueous solution containing acetic acid or its salt, acetic acid is released to the atmosphere when the water-containing pellets resulting from the contact are dried. Therefore, the surrounding environment and the working environment may be affected.
The present invention was created for the purpose of solving the above-mentioned problems. The purpose of the present invention is to provide a method for producing an EVOH resin composition and a method for producing EVOH resin composition pellets which are environmentally friendly and in which no carboxylic acid such as acetic acid is released to the ambient atmosphere.
Another object of the present invention is to provide an EVOH resin composition and EVOH resin composition pellets which emit little smell and suitable, for example, for food packaging applications, and more preferably to provide an EVOH resin composition and EVOH resin composition pellets which have an improved melt stability and also have a superior long-run workability.